1. Field of the Invention
The present invention relates to a heat exchanger that constitutes a portion of a cooling cycle, and in particular, it relates to a heat exchanger constituted by a pair of header pipes with a plurality of flat tubes communicating between the header pipes, and with the bonding of the header pipes and the flat tubes achieved by inserting and brazing the flat tubes at the header pipes.
2. Description of the Related Art
A heat exchanger constituted by a pair of header pipes with a plurality of flat tubes communicating between the header pipes is employed as a condenser or the like for cooling a high pressure coolant, and an example of such heat exchanger is disclosed in Japanese Unexamined Patent Publication No. H8-145591. In this heat exchanger, which is constituted by adopting a prerequisite bonding structure in which end portions of the flat tubes are inserted into tube insertion holes formed in the header pipes for brazing, a brazing margin similar to those at the side edges is formed at a middle area of each tube by dividing the end portions of the flat tube into separate portions, and the insertion holes of the header pipes are formed in a shape matching the shape of the end portions of the flat tubes to improve the brazing characteristics of the flat tubes themselves.
However, while the structure at the side edges is replicated at the middle area of each end portion according to the invention described above, based upon the observation that the brazing is achieved in a reliable manner at the aligned side portions of the flat tubes along their edges at the ends, it is also necessary to achieve good brazing for the flat tubes and the header pipes as well as to achieve good brazing for the flat tubes themselves. In particular, since the heat exchanging medium does not leak out of the tubes as long as good brazing is achieved at the side edges of the flat tubes, defective brazing at individual ridges within the flat tubes does not present a major problem. Rather, the brazing state between the header pipes and the flat tubes is a more critical concern, and if defective brazing occurs between the header pipes and the flat tubes, the heat exchanging medium may leak, and this will constitute a fatal defect in the heat exchanger. Consequently, priority should be given to eliminating any risk of defective brazing between the header pipes and the flat tubes.
Reviewing the prior art technology described above from this point of view, concerns arise in that forming the middle area of each end of a flat tube in an identical shape to that at the side edges and forming the insertion holes of the header pipes in a complicated shape to match this may reduce the degree of efficiency in the assembly work when a plurality of flat tubes are inserted at the insertion holes of the header pipes and in that, since the shape of the insertion holes is complicated, gaps between the header pipes and the flat tubes are more likely to be inconsistent, thereby reducing the yield of the brazing material.
Thus, it is desirable to form the end portions of the flat tubes to be inserted into the header pipes, and the tube insertion holes themselves, in as simple a shape as possible. Based upon this objective, the applicant of the present invention has previously proposed a bonding structure in which the end portions of the flat tubes are flattened and the tube insertion holes are also constituted of sides that are made as linear as possible in conformance to the shape of the flattened end portions to assure good brazing for the header pipes and the flat tubes.
As illustrated in FIG. 7, in this bonding structure, flat tubes .alpha. are each achieved by folding over a single brazing sheet to form a bend along the lengthwise direction with the two side edges facing opposite each other brazed to form heat exchanging medium passages, a ridge .gamma. is formed to extend along the lengthwise direction at a flat portion of the flat tube a that comes in contact with a fin .beta. and the ridge .gamma. is placed in contact with the inner surface of the flat portion on the opposite side to improve the pressure withstanding performance and the strength of the flat tube .alpha.. In particular, the structure is characterized in that no ridge .gamma. is formed at the end portions of the flat tube .alpha. where it is to be inserted into insertion holes .epsilon. in header pipes .delta. and that the end portions of the flat tube .alpha. is formed flat with the insertion holes .epsilon. formed in a simple shape to match.
While good brazing is assured for the header pipes and the flat tubes in this structure, since no ridge is formed at the end portion of the flat tubes .alpha. to be inserted at the insertion holes .epsilon., it is necessary to reinforce these areas. In this case, if the ridge .gamma. is to be simply extended to be present at the end portion to achieve reinforcement, the shape of the insertion holes .epsilon. at the header pipes .delta. must be complicated to match the surface shape at the end portions of the flat tubes .alpha., as in the prior art technology explained above, which will tend to result in brazing defects between the header pipes .delta. and the flat tubes .alpha..